1. Field of the Invention
The instant disclosure relates to a power metal-oxide-semiconductor field-effect transistor (MOSFET) and manufacturing method thereof; in particular, to a trench power MOSFET having a bottom dielectric and a manufacturing method thereof.
2. Description of Related Art
Power metal-oxide-semiconductor field-effect transistors (Power MOSFET) are widely implemented in the switching devices of electric devices, such as power supplies, rectifiers or low voltage motor controllers and the like. The current power MOSFET is designed to have a vertical structure to improve the packing density. The power MOSFET having trench gate structure not only results in higher packing density, but also has lower on-state resistance. The power MOSFET having vertical structure is also called the trench power MOSFET. One of the advantages of the trench power MOSFET is that it is capable of controlling the operation of devices with low-power consumption.
The parameters affecting the characteristics of the power MOSFET include source-drain on-state resistance (Rdson), breakdown voltage and switching speed. However, the source-drain on-state resistance is positively correlated to the breakdown voltage. That is, the increase of the doping concentration of the drift region improves lower source-drain on-state resistance but sacrifices high breakdown voltage. Accordingly, many solutions have been developed to balance the charge of the drift region in order to reduce the source-drain on-state resistance without sacrificing the breakdown voltage.
One of the solutions is the bottom portion of the gate trench is filled with a thicker bottom oxide layer. However, the aspect ratio of the gate trench increases as the cell density and the breakdown voltage increase. Please refer to FIG. 1A and FIG. 1B, which respectively illustrate cross-sectional diagrams of a power MOSFET in different steps during the prior art fabricating method.
As shown in FIG. 1A, after the trenches 111 are formed in the epitaxial layer 110, an oxide 112 fills each of the trenches 111, to be formed on the bottom and the side walls of each of the trenches 111. However, an overhang of the oxide 112 is easily formed near an opening end of the trench 111 because the oxide 112 has poor step coverage. The overhang hangs over the remaining unfilled portion of the trench 111 forming a neck in the unfilled portion of the trench where the upper part of the unfilled portion of the trench 111 is narrower than the lower part of the unfilled portion of the trench 111, such that a void 113 is formed in the trench 111. Please refer to FIG. 1B. Subsequently, a portion of the oxide 112 formed on a top surface of the epitaxial layer 110 and an upper portion of the trench 111 is removed by etching process. The remained bottom oxide 112′ located at a lower portion of the trench 110 still has a void 113′ formed therein. It is likely that the void 113′ extends from the upper surface to a bottom of the trench 111.
During the following deposition process of the gate structure in the trench 111, the gate structure fills the void 113′ and may be directly in contact with the epitaxial layer, which would result in poor electrical performance of the power MOSFET.